This invention relates generally to the field oil wells, and more particularly relates to a method and apparatus for pumping oil from an oil well.
A wide array of techniques for extracting fluids, such as oil, from deep wells have been developed and practiced over the years. One well-known method for the recovery of deep well fluids, especially in the context of oil wells, utilizes a so-called xe2x80x9cwalking beam pump jack,xe2x80x9d sometimes referred to more generically as a xe2x80x9crod-and-beam-pump.xe2x80x9d A typical rod-and-beam pumping system has a sucker rod string attached to one end of the beam, with the beam being driven by a prime mover, usually a motor, through belts, pulleys, and a gear box system coupled to the opposite end of the beam by means of a pitman arm. The sucker rod string extends down into the well and is connected to a down-hole pump. The prime mover causes the pitman arm to provide a rocking motion of the walking beam, raising and lowering the sucker rod string. This vertical oscillation of the sucker rod string results in the lifting or sucking of fluid out of the well.
Whereas the rocking motion of a walking beam is provided by the prime mover, the resulting forces counterbalance the weight of fluid being lifted. The load on the prime mover is at a maximum when the end of the walking beam to which the sucker rod string is coupled begins its upward stroke. The load includes the weight of the sucker rod string, the weight of the fluid being lifted out of the well, and the force needed to overcome the inertia of the load following the downward stroke of the sucker rod end of the beam. At the point of maximum load on the prime mover, the constant load permits the sucker rod to reach a constant velocity until it approaches the top limit of the upward stroke. Upward movement decelerates, then ceases, followed by a subsequent downward stroke. The weight of the sucker rod string accelerates the downward movement of the walking beam until the sucker rod reaches the bottom of its down stroke, thus completing a complete pumping cycle.
The loads imposed on the prime mover of an oil well pump jack are considerable. During the upstroke of the sucker rod end of the beam, in a typical 5000 foot (1524 m) deep well, the weight of the sucker rod and the oil being lifted can reach approximately eight thousand pounds (2639 kg).
While generally popular, there are some potential disadvantages of conventional walking beam pumping systems. Conventional pumping systems are not known to be particularly efficient. Moreover, as a result of the various forces at play, severe stresses are induced into many of such systems"" mechanical components, resulting in significant maintenance requirements and undesirable and costly periods of inoperability.
In view of the foregoing considerations, the present invention is directed to an improved method and apparatus for walking beam pumping. In one embodiment, an oil well pumping system is provided in which a walking beam is mounted on a beam support structure, for example, a heavy steel I-beambase or a concrete base. A sucker rod string is coupled to one end of the walking beam, and a pitman arm connects the opposite end, referred to as a drive limb in conventional walking beam pumping systems, to a rotating crank arm. Unlike conventional systems, however, the crank arm is not connected to a gearbox, but instead merely serves to limit the range of walking beam movement.
In accordance with one aspect of the invention, no mechanical force is delivered to the pumping system through the crank arm and pitman rod. Instead, the prime mover of the system is gravity, as modulated by means of a suitable counterbalance weight moveable along the length of the walking beam and carried on a low-friction bearing or the like. Moving the counterbalance weight away from the pivot point at which the walking beam is coupled to the support structure in the direction of the pitman rod increases the amount of lifting force exerted on the sucker rod string at the opposite end of the walking beam, resulting in the lifting or pumping of fluid out of the well. Conversely, moving the counterbalance weight closer to the sucker rod string end of the walking beam reduces the lifting force exerted on the sucker rod string, allowing the sucker rod string to descend back into the well in preparation for a subsequent upstroke.
Through careful dynamic modulation of the positioning of the counterbalance weight along the length of the walking beam, the beam position, motion, and sucker rod string stress can be controlled. Movement and positioning of the counterbalance weight can be accomplished by several means, including but not limited to linear electric motors, lead screws, and/or hydraulic actuators.
In accordance with another aspect of the invention, a stationary weight may be affixed on the drive limb end of the walking beam to balance the system and minimize the necessary weight of the counterbalance weight. In an optimal implementation, the counterbalance weight need only be heavy enough to displace the lifted fluid. Shifting the counterbalance weight away from the sucker rod string end of the beam increases lifting force, thereby increasing the stress placed on the sucker rod string. Determining the desired positioning of the counterbalance weight can be accomplished through analysis of its previous positioning.
In accordance with another aspect of the invention, sensors and feedback devices may be provided to determine walking beam positioning and velocity. This data, combined with the known positioning of the counterbalance weight, can be used to calculate the amount of fluid being lifted out of the well, the fluid level in the well, and even oil/water content of the fluid being extracted.
In accordance with still another aspect of the invention, certain down-hole conditions may also be maintained through automatic adjustment of the pump cycle rate. For example, the actual weight in air of the sucker rod string and all other parts of the system are known quantities. The resulting weight of the sucker rod string in fluid is computed as the weight of the sucker rod string in air minus the volume of the rod string that extends below the fluid level in the well times the density of the fluid. The total resultant weight of the rod string varies only with pump stroke and density of fluid. At some point in each pump cycle, based upon beam position, beam velocity, and counterbalance weight position, the weight and xe2x80x9cheadxe2x80x9d of the fluid column can be determined and reported to the control system. Further, through calibration, the fluid level of the well and fluid density can be accurately determined.
When the walking beam is at the top of its stroke, a determined amount of sucker rod string is submerged in fluid. The counterbalance weight can then be moved to a position that balances the system, such that there is no movement of the walking beam. Determination of this xe2x80x9cneutralxe2x80x9d position for the counterbalance weight enables the weight of the sucker rod string in the fluid, since (1) the system is at rest; (2) no fluid is being lifted; and (3) there is no working fluid head. Hence, an accurate assessment of fluid density can be made. If fluid density can be accurately determined, head pressure can also be determined by measuring conditions during the up (or pump) stroke. From fluid density and head pressure values, fluid level in the well can be derived. Those of ordinary skill in the art will appreciate that all such information is important to achieve optimum output of usable fluid from a well. Pumping too fast results in either dropping the fluid level of the well or increasing the ratio of water to oil. Being able to accurately determine the down hole conditions in accordance with the principles of the present invention can significantly increase well efficiency.